1. Field of the Invention
The invention relates to a mass flowmeter for flowing media which works on the Coriolis principle, with at least one straight measuring tube conveying the flowing medium, with at least one oscillation generator acting on the measuring tube, with at least one measurement value sensor detecting Coriolis forces and/or Coriolis oscillations based on Coriolis forces and outputting a measurement signal, with a supporting tube accommodating the measuring tube, the oscillation generator and the measurement value sensor, with a stress sensor for detecting the stress state of the measuring tube and with a correction device for correcting the measurement signal, whereby the measuring tube and the supporting tube are connected to one another in a manner excluding relative axial movements and the axial spacing of the fixing points of the supporting tube to the measuring tube represents the oscillation length of the measuring tube and whereby the measurement value sensor and the stress sensor detecting the stress state of the measuring tube are connected to the correction device, in order to feed to the correction device the measurement signal and the stress signal outputted by the stress sensor detecting the stress state of the measuring tube.
The invention further relates to a method for correcting the measurement signal of a mass flowmeter for flowing media, which works on the Coriolis principle and has at least one straight measuring tube conveying the flowing medium, at least one oscillation generator acting on the measuring tube, at least one measurement value sensor detecting Coriolis forces and/or Coriolis oscillations based on Coriolis forces and outputting a measurement signal and a supporting tube accommodating the measuring tube, the oscillation generator and the measurement value sensor, whereby the measuring tube and the supporting tube are connected to one another in a manner excluding relative axial movements and the axial spacing of the fixing points of the supporting tube to the measuring tube represents the oscillation length of the measuring tube and whereby the stress state of the measuring tube is detected.
With mass flowmeters for flowing media which work on the Coriolis principle, so-called Coriolis mass flowmeters, a distinction is basically made between, on the one hand, devices whose measuring tube is designed curved, e.g. loop-shaped, and on the other hand, devices whose measuring tube is essentially straight. Furthermore, a distinction is made with the Coriolis mass flowmeters in question between, on the one hand, those that have only one measuring tube, and on the other hand, those that have two measuring tubes. In the case of the forms of embodiment of the Coriolis mass flowmeters with two measuring tubes, these can lie in a row or parallel to one another from the flow technology standpoint.
Forms of embodiment of Coriolis mass flowmeters with which the measuring tube is designed straight, or with which the measuring tubes are designed straight, can, in view of the mechanical structure, be produced simply and consequently at relatively low cost. The Coriolis mass flowmeters obtainable in this way are compact and lead to only low pressure loss.
The drawback with such Coriolis mass flowmeters with which the measuring tube is designed straight, or with which the measuring tubes are designed straight, is that both length changes of thermal origin and stresses of thermal origin as well forces and moments acting from outside lead to measurement errors and to mechanical damage, i.e. to stress cracks.
2. Description of the Prior Art
A mass flowmeter and a method for correcting the measurement signal of a mass flowmeter, as described at the outset, are known for example from DE 42 24 397 C1. With the Coriolis mass flowmeter described there, there is provided, as a stress sensor for detecting the stress state of the measuring tube, a length-change sensor which detects changes in the oscillation length of the measuring tube in order to correct the measurement signal in dependence on the oscillation length and the stress.
Due to the fact that a length-change sensor detecting changes in the oscillation length of the measuring tube is provided in the case of this Coriolis mass flowmeter known from the prior art, a change in the oscillation length and in the axial stress state of the measuring tube influencing the oscillation frequency of the measuring tube can be taken into account, as a result of which measurement errors can be reduced or eliminated. With the additional provision of a temperature sensor, it is possible to reduce or eliminate measurement errors based on temperature changes of the measuring tube on the one hand and those based on forces acting on the measuring tube from the outside on the other hand. The length-change signals originating from the length-change sensor are a direct measure of changes in the oscillation length of the measuring line, irrespective of their origin, and an indirect measure of changes in the axial stress state of the measuring tube, also irrespective of their origin. The length-change sensor for detecting the changes in the oscillation length of the measuring tube thus makes it possible to detect changes in the oscillation length of the measuring tube and changes in the axial stress state of the measuring tube and to reduce or eliminate errors based thereon in the measurement signal when determining the value of the mass flow rate.
As far as the measurement errors arising due to temperature changes are concerned, the following further applies: the temperature dependence of the modulus of elasticity influences the oscillation frequency and the flexibility of the measuring tube and thus the measurement signal outputted by the measurement value sensor. As a result of this knowledge, a temperature sensor detecting the temperature of the measuring tube is provided for the measurement-signal correction dependent on the measuring-tube temperature. In this regard, reference is also made to the article “Direct mass flow rate measurement, in particular with the Coriolis method” by W. Steffen and Dr. W. Stumm in “measurement, testing and automation”, 1987, pp. 301-305.
Furthermore, a Coriolis mass flowmeter is known from the prior art, with which the ongoing temperature dependence of the measurement signal is taken into account by the fact that a temperature sensor detecting the temperature of the supporting tube is provided for the measurement-signal correction dependent on the supporting-tube temperature. This is described in DE 36 32 800 A1 and in EP 0 261 435 B1. The correction measures described there make provision such that the temperature-sensor signals generated by the two temperature sensors are inputted into a correction device which is intended to remove the temperature influence on the measurement signal.
All the previously described devices and measures that have been taken with Coriolis mass flowmeters to obtain a measurement-signal correction dependent on stress and temperature have led to an improvement in the ascertainment of the mass flow rate signal. The known measures are not, however, fully satisfactory, since it emerges that the ascertained mass flow rate signals continue to be bound up with errors, even though small ones.